(1) Background of the Invention
The present invention relates to capacity adjustment apparatus and method of secondary batteries.
(2) Description of Related Art
In assembled cells in which a plurality of cells (secondary batteries) are connected, differences in the capacities (residual capacities, hereinafter, described merely as capacities) of the respective cells occur due to variations in characteristics of the respective cells when charges and discharges are repeated and when the assembled cells are left alone. In a state where the assembled cells are used when such capacity differences as described above occur, some cells in states of overcharges and overdischarges are developed so that a life of a whole of the assembled cells becomes short. Therefore, uniformities of capacities of the respective cells are carried out at predetermined frequencies.
In a lithium ion secondary battery in which a lithium cobalt oxide is used for a positive pole and a carbon is used for a negative pole and in a lithium secondary battery in which a lithium cobalt oxide is used for the positive pole and a lithium metal is used for the negative pole (hereinafter, these lithium ion secondary b battery and lithium secondary battery are, in a general term, also called a lithium series secondary battery), such an organic solvent as ethylene carbonate is used for an electrolyte. Hence, if the lithium series secondary battery overcharges, the organic solvent is decomposed and gasified. A frame of each secondary battery is expanded and the organic solvent which is the electrolyte is gasified. Hence, when the subsequent charge is carried out, a charge capacity is extremely reduced due to a full capacity thereof caused by a deterioration of the secondary battery.
To cope with the above-described problem, in the assembled cells of the lithium series secondary battery, such a method in which the capacity of each cell is uniformed by discharging the cells having larger capacities (residual capacities, namely, the residual electric powers) than the other cells has been adopted. For example, a Japanese Patent Application Publication No. 2003-284253 published on Oct. 3, 2003 discloses a method of calculating the adjustment quantity of each cell from a deviation between open voltage values of the individual cells and 1/x of a voltage value of a minimum voltage value B added to a maximum voltage value A from among the open voltage values (x denotes a constant between (A+B)/A and (A+B)/B). It should be noted that the capacity adjustment of the cell is carried out by discharging a capacity adjustment bypass resistor connected in parallel to each cell by a time duration corresponding to an adjustment capacity.